Heretofore, a mobile communications system, which controls a downlink packet transmission, by adaptively changing a downlink packet transmission method (for example, such as a transmission format of a modulation scheme, a coding rate, or the like) in accordance with a downlink radio condition when a radio base station performs communications with mobile stations belonging to the radio base station. Such a control system is called an “AMC (Adaptive Modulation and Coding)” system.
In a mobile communications system to which such an AMC system is applied, a mobile station is configured to monitor downlink radio conditions, and to notify the radio base station of the monitored downlink radio conditions by using an uplink. Here, the downlink radio conditions include an SIR, a CIR, a received power, and the like.
Furthermore, the radio base station is configured to determine a downlink transmission method (for example, such as a transmission format of a modulation scheme, a coding rate, or the like) on the basis of the downlink radio conditions notified by the mobile station, and a transmission resource (a radio resource) available for the downlink packet transmission, and then to perform the downlink packet transmission with the determined transmission method.
At this time, in a case where the communications between the radio base station and the mobile station can be performed according to the downlink radio conditions, that is, in a case where the downlink radio conditions are good as in such a case where the mobile station is located near the radio base station, and a case where the moving speed of the mobile station is low, the mobile communications system to which the AMC system is applied is configured to perform the downlink packet transmission by use of a transmission method capable of performing communications at a higher transmission rate.
On the other hand, in a case where the downlink radio condition is poor as in such a case where the mobile station is located at the edge of the cell, and a case where the moving speed of the mobile station is high, the mobile communications system to which the AMC system is applied is configured to perform the downlink packet transmission by use of a transmission method capable of performing communications at a lower transmission rate.
Thus, the mobile communications system to which the AMC system is applied becomes capable of performing efficient communications in accordance with a change in a propagation environment.
Incidentally, as to the standardization of the Third Generation Mobile communications system, so called “IMT-2000” in the 3GPP/3GPP2 (Third-Generation Partnership Project/Third-Generation Partnership Project 2) organized by a regional standardization organization, or the like, the groundwork is laid for the standard specification for “W-CDMA system” in the former, and the groundwork is laid for the standard specification for “cdma2000 system” in the later.
In the 3GPP, on the basis of a prediction that high speed and large capacity traffic by downloading from a data base or a Web site specifically in downlink increases accompanying by the rapid widespread of the internet in recent years, the standardization of the “HSDPA (High Speed Downlink Packet Access) system”, which is a downlink high speed packet transmission system, has been in process.
Furthermore, also in the 3GPP2, from a viewpoint similar to the aforementioned one, the standardization of a downlink high-speed packet transmission system, “1x-EV DO” has been in process. Note that “DO” means “Data Only”” in the “1x-EV DO” of the cdma2000 system.
For example, in the HSDPA system, in accordance with the radio conditions between a mobile station and the radio base station, the AMC system which controls the modulation scheme or the coding rate of a radio channel is used. Here, the mobile station is configured to notify the radio base station of the downlink radio condition, by transmitting control information (radio condition information) called, CQI (Channel Quality Indicator) through the uplink. Note that the CQI is subjected to mapping with an uplink HS-DPCCH (High Speed-Dedicated Physical Control Channel) for the HSDPA.
Moreover, the mobile station is configured to calculate the aforementioned CQI on the basis of an SIR found from a downlink common pilot channel (CPICH: Common Pilot Channel). For example, the mobile station is configured to calculate the CQI so that the error rate of the received packet becomes 10%.
On the other hand, the radio base station determines, by use of the CQI and a downlink radio resource (that is, a power resource and a code resource), a transmission method (specifically, a transmission format or a transmission resource such as a modulation scheme, an amount of a code resource, a transport block size (TBS), or an offset value for an amount of a power resource), the method being used in the downlink packet transmission, and then, performs the downlink packet transmission by use of such a transmission method.
Furthermore, in the HSDPA system, the radio base station and a mobile station performs retransmission control, which is called H-ARQ control, at the same time. The mobile station receives a downlink packet, causes the decoding result (OK or NG) of the received packet to be subjected to mapping with an HS-DPCCH, and transmits it through the uplink as acknowledgement information regarding the downlink. On the other hand, the radio base station performs retransmission control by the H-ARQ control on the basis of the acknowledgement information.
Here, as the acknowledgement information regarding the downlink, there is Ack/Nack/DTX. The “Ack” is an affirmative response (OK) indicating that the receiving of a packet has been successful. The “Neck” is a negative response (NG) indicating that the receiving of a packet has failed. The “DTX” indicates that the mobile station has failed to receive downlink shared control information HS-SCCH in the downward direction for some reason, and has not received the downlink packet.
However, in the conventional mobile communications system, there has been a problem that it is difficult to control the trade-off relationship between reduction in a transmission resource used in a packet transmission and deterioration of an error rate.
Moreover, in the conventional mobile communications system, there has been a problem that a transmission resource (a radio resource such as a code resource or a power resource) is unnecessarily consumed since the transmission method used in a downlink packet transmission is not determined on the basis of the difference between a radio channel quality at the time of a retransmission by the H-ARQ control and a radio channel quality at the time of a initial transmission, or of the amount of data of the packet to be transmitted.